The invention applies notably in the field of satellite transmissions. It can also be used in other cases of employment such as sporadic burst transmissions in a multi-path channel. The use of hybrid preamble (reference+subset) makes it possible notably to substantially lengthen the preamble while improving the decoding of the bits transported by the hybrid preamble.
The invention is also used for continuous transmission modes.
The invention can also be beneficial in the case of fast-evolving channels. Specifically, these channels require fast tracking of the channel, which is conventionally obtained by repeating the preambles with a small period.
The use of powerful codes, such as turbo codes, makes it possible to reduce the values of the signal-to-noise ratio denoted by the abbreviation SNR that are necessary for transmitting a message. This reduction brings demodulators ever nearer to the limits of their channel estimation capability. This is all the more true as the decrease in SNR allows a rise in spectral efficiency related to the use of more complex constellations, for example, the 16 QAM constellation (the abbreviation standing for quadrature amplitude modulation), the 32 APSK constellation (the abbreviation standing for amplitude phase shift keying), etc. The coding schemes obtained, often BICMs (the abbreviation standing for Bit Interleaved Coded Modulation) make it possible to get still closer to Shannon's theoretical limit, assuming that the modem behaves optimally.
The demodulator must estimate parameters such as the arrival amplitude, the phase, the incident frequency, the multi-path channel, etc. This is all the more difficult as the noise increases.
The currently known methods for solving this problem consist, for example, in periodically adding references at the signal emission level. These references are disposed in blocks or in a discrete manner.
When the demodulation conditions are difficult, such as for example for demodulations in short bursts (burst, TDMA) or when the channel is difficult to demodulate (multi-path channel, considerable phase noise, considerable frequency error or Doppler acceleration), the emission periodically inserts, notably at the start (burst satellite) or in the middle of a burst (GSM or global system mobile), known symbols, called pilot symbols or reference symbols. The insertion of these reference symbols increases the band used and the ratio Eb/N0 corresponding to “the energy per information bit transmitted” and “the white noise spectral density”.
When the reference symbols are not sufficiently numerous, the demodulator makes estimation errors which result in noise added to the noise received by the modem. This demodulation noise can degrade the performance of the demodulator or even lead to a dropout of the latter which no error code can remedy. Conversely, when the reference symbols are too numerous there is a loss in terms of spectral efficiency (the inserted symbols do not carry any information) and signal-to-noise ratio (the energy invested in the reference symbols is not used by the decoder).
One of the drawbacks of these techniques is therefore that they make an important difference between the data symbols and the reference symbols.